The invention relates to a control device for an engine-independent heater that heats a liquid heat transfer medium of a heat transfer medium circuit especially for motor vehicles, the control device reducing a heating capacity of the heater when a current temperature gradient of the heat transfer medium is equal to or greater than a temperature gradient threshold value.
The invention further relates to a heater and a heating system.
Moreover, the invention relates to a method for controlling an engine-independent heater that heats a liquid heat transfer medium of a heat transfer medium circuit especially for motor vehicles.
There are already known in the prior art measures for the early detection of an overheating risk for a heater that is integrated in a heat transfer medium circuit. The aim in this case is to detect the threatening overheated state clearly before the occurrence of the actual overheated state such that it is possible to adopt countermeasures before damage occurs to the heater or the heat transfer medium circuit.
DE 195 24 260 C2 discloses a control device for an engine-independent heater, a heater, a heating system and a method for controlling an engine-independent heater. The heater described in DE 195 24 260 C2 is used especially for heating the interior of a motor vehicle. During operation, in the case of this heater the heating capacity is reduced or the heater is switched off when a current temperature gradient of the heat transfer medium overshoots a threshold value of the temperature gradient. Various threshold values of the temperature gradient are selected as a function of the current temperature of the heat transfer medium. Thus, a comparatively fast temperature rise can be permitted without a risk of overheating as long as the temperature level remains low and thus clearly below a dangerous temperature level. By contrast, as soon as high temperatures have already been reached only a comparatively slow temperature rise is still permissible.
Since, consequently, the threshold values of the temperature gradient are assigned fixed temperature values of the heat transfer medium, this assignment must be stipulated in advance as a function of the properties, such as thermodynamic properties, for example, of the heat transfer system. After a change of the heat transfer system, for example after the use of another heat transfer medium, the control of the heater can now only be used conditionally and is no longer adapted to the changed system.